My roommate has a bigger tank for her betta that came with a divider. We've put my betta in on the other side and her betta constantly flares at mine. Is it healthy to put a see-thru divider in if the betta constantly flares? Would that cause stress?

Let me be the one to caution against this. My boys were fine with the see through divider at first.. only flaring occasionally. Then thats all they would do is patrol the divider constantly flaring. Eventually they both got so stressed out that they bit their tails to shreds (meaning weeks of treatment for me).

What I would do is take the clear divider out and put in a piece of plastic mesh... they can still see a little but not enough to really stress them out.